1. Field
The present invention relates to a lithographic apparatus having a feedthrough control system, and a method for manufacturing a device.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The manufacture of ICs and other devices with a lithographic apparatus generally involves the replication of extremely fine sub-micron patterns, with an exceptionally high degree of positional accuracy. For this reason, it is desirable to properly isolate various critical parts of the apparatus (such as the substrate table (substrate support) and mask table (patterning support), for example) from spurious motion, vibration, mechanical shocks, etc. In general, this is achieved using such measures as carefully designed metrology frames, air-mounts, motional counterweights and dampers, which serve to isolate the critical parts of the apparatus from most unwanted mechanical influences. However, such measures are not completely effective in eliminating a number of unwanted influences, such as, for example:                1. vibrations in the substrate table due to leveling actions during exposure;        2. vibrations caused by motion of reticle masking blades;        3. resonance effects caused by the presence of air showers;        4. vibrations in the substrate support caused by motion of the patterning support, and vice versa; and        5. influence of air shower flow on the substrate support.        
Although these effects are relatively small, they become increasingly important as the need to produce ever-higher device resolutions increases, and they form a substantial barrier to the viable realization of large-area ICs having critical dimensions of the order of 0.05 μm or less.
Accordingly, it has been proposed in U.S. Pat. No. 6,373,072, which is incorporated by reference herein, to provide a control system for the substrate and patterning supports of a lithographic apparatus in which errors in the position of the substrate support are compensated for by their inclusion as a feedforward control in the patterning support control loop. Specifically, the substrate support error is low-pass filtered, and the output of the filter is then added to the patterning support setpoint. The substrate support error is also twice differentiated, and multiplied by the patterning support mass, and the resultant force is applied to the patterning support. The proposal of this prior art is based on the realization that the absolute positions of the patterning and substrate supports are less important than their relative position and allows the correction of substrate support errors beyond the patterning support bandwidth. However, this control system has performance limits.
A major limitation in the achievable performance of the known control system is the delay in the feedthrough path processing the substrate support error, as is illustrated with reference to FIGS. 2 and 3.
FIGS. 2 and 3 schematically illustrate a prior art feedforward control system constructed and arranged to control movement of a patterning support MT for a patterning device MA by reference to a measured state of the substrate support WT, where a radiation beam patterned by the patterning device MA passes through a projection system PS to reach a substrate W.
The arrangement shown in FIG. 2 includes a measurement system 2 configured to measure the state (e.g., position, velocity, acceleration and/or further time derivatives of the position) of the substrate support WT. A feedforward control system 4 is provided for controlling the movement of the patterning support MT via force actuator 6. The control system 4 will be described in more detail below. Connections within control system 4 are illustrated in FIG. 3. The position of the patterning support MT may be measured using a measurement system 8. Each measurement system 2, 8 may include an interferometer.
FIG. 3 shows a block diagram illustrating the principles underlying the feedforward control system 4 of FIG. 2. A top control loop 20 represents the patterning support MT (or reticle stage RS) including a mechanical transfer HRS and controller CRS. A bottom control loop 22 represents the substrate support WT (or wafer stage WS), including the mechanical transfer HWS and controller CWS. Each of the control loops 20 and 22 include a feedback loop 24, 26, respectively, feeding back a measurement of the respective patterning support position yRS and the substrate support position yWS to the inputs of the respective control loops 20, 22. The mechanical transfers HRS and HWS include all aspects of a transfer function from a force generated by the respective controllers CRS, CWS, to an actual movement of the respective patterning support MT and substrate support WT. However, in the bottom control loop 22 a block IFM is shown explicitly to indicate that the mechanical transfer HWS includes (part of) a position measurement system, e.g. an interferometer (IFM) system, to determine the substrate support position yWS, having a sampling frequency of e.g. 20 kHz and a data output frequency of e.g. 5 kHz. A similar position measurement system may be included in the mechanical transfer HRS of the top control loop 20 to determine the patterning support position yRS. Between the bottom control loop 22 and the top control loop 20, a feedthrough path is provided.
The top control loop 20 of the patterning support MT receives L times a position setpoint SP POS of the bottom control loop 22 of the substrate support WT, while its output only counts 1/L in the relevant WS/RS error eWRS, reflecting the fact that a pattern image is projected by the projection system PS with a magnification 1/L, and that the patterning support MT scans at L times the speed of the substrate support WT. For example, L may have a value between 3 and 6, e.g. 4.
The control system is arranged to feed an error eWS (defined as the difference between an external setpoint SP POS and a measured position yWS of the substrate support WT) from the substrate support control loop 22 to the patterning support control loop 20 by means of two signal branches 30 and 32. The first branch 30 is configured to multiply error eWS by a factor K in a stage 34. The factor K may be arranged to be proportional to the projection system magnification (in this case L times) in order to create a movement of the patterning support MT that is L times as large as that of the substrate support WT. In addition, also in stage 34, low-pass filtering by a filter 1/D is applied. The second branch 32 is arranged to doubly differentiate the substrate support control error eWS, in a stage 36, in order to create a required acceleration of the patterning support MT. Stage 36 is then configured to multiply this acceleration by the patterning support mass m and the factor K and, finally, apply the same low-pass filtering 1/D that was applied in stage 34 of branch 30. The design is such that movement of the patterning support MT as a reaction to the feedforward branch 32 is such that it matches the addition to the input of the patterning support control loop 20, the output of stage 34.
In other words, the second branch 32 is configured to calculate the required force on the patterning support MT such that it moves in accordance with the signal from the first branch 30 added to the position setpoint SP POS. The aim of this arrangement is to ensure that the control error eRS of the patterning support remains zero. When applied on its own, the controller CRS would consider the force from the second branch 32 a disturbance, and would react and try to diminish the effect of the feedforward. By additionally feeding the filtered error eWS to the patterning support controller CRS, this problem is solved. Now, if the patterning support MT reacts to the extra feedforward force as a transfer 1/ms2, the error eRS remains zero and hence the controller CRS is left “unaware” of any extra force injection.
The feedthrough path, including the branches 30 and 32, to the actual measured displacement yRS suffers from a delay. A minimum delay of 1.5 samples in the digital signal processing occurs in the path from eWS to the patterning support response (in reality it is closer to 2 samples). One sample thereof is caused by the double differentiation in the feedforward path (branch 32). This can be seen when considering the digital form of differentiation of a position signal. A first derivative v(k), velocity at a time k, is calculated as:
      v    ⁡          (      k      )        =            1              T        S              ⁢          (                        e          ⁡                      (            k            )                          -                  e          ⁡                      (                          k              -              1                        )                              )      and a second derivative a(k), acceleration at a time k, is calculated as:
      a    ⁡          (      k      )        =            1              T        S        2              ⁢          (                        e          ⁡                      (            k            )                          -                  2          ⁢                      e            ⁡                          (                              k                -                1                            )                                      +                  e          ⁡                      (                          k              -              2                        )                              )      Here, Ts is a sampling time period, and e(k) is the signal that needs to be differentiated. As the calculation of derivatives uses past data, the calculated derivative lags behind the actual derivative by 0.5 sample for a first derivative v(k), and 1 sample for a second derivative a(k). The other 0.5 sample occurs due to the zero-order hold in the patterning support control loop 20 (more particularly, HRS). This delay stems from the fact that the controller output will remain on digital-to-analog converters for a complete sample period, and hence the average data age is 0.5 sample. More delay is caused by calculation time needed in the controllers CRS and CWS, and some additional filtering, which is not discussed in detail here. This means that the patterning support response to the substrate support position error eWS lags behind at least 1.5 samples, and hence the correction provided by the feedthrough is out of phase with regard to the substrate support position error eWS. Especially for higher frequencies this effect reduces the performance of the lithographic apparatus.
It is to be noted here that the patterning support holds a patterning device, which is an optical component, i.e. a component which is part of an optical system of a lithographic apparatus. Accordingly, any control of a positioning of the patterning support as described above may equally or additionally be applied to a positioning of another optical component of the optical system of the lithographic apparatus, achieving the same or similar effects. Therefore, any reference in this specification to positioning a patterning support is equally or additionally valid as a reference to positioning an optical component of the lithographic apparatus, such as a part of an illuminator or a projection system.
Applying the prior art feedthrough control, as an indication of the average position error of a point on the substrate during an exposure time, MA (Moving Average) error, may be improved by a factor of about two. An image contrast loss (fading) expressed as an MSD value (Moving Standard Deviation), may only be improved slightly.